Patent GB-787425-A (GB)
Title: Improvements to television switching equipments
IPC: H03K17/54
Filed: 1954-02-15
Abstract: 787,425. Television. SOC. NOUVELLE DE L'OUTILLAGE R.B.V. ET DE LA RADIOINDUSTRIE. Feb. 15, 1954 [Feb. 16, 1953; March 16, 1953; Nov. 4, 1953; Dec. 29, 1953], No. 4407/54. Class 40 (3). In an arrangement for feeding a desired one of a plurality of synchronized separate video signal sources (e.g. separate television cameras concerned with different scenes of a single broadcast transmission) to a transmission channel, the desired source is first selected and " change-over " to this source is then effected automatically during a (subsequent) vertical blanking period. Fig. 1 shows the general arrangement in which operation of a selected one of push-button switches I1, I2, I3 (the operation of one releasing the others) conditions the appropriate one of electronic switch devices 5, 6, 7 so that on the occurrence of a pulse A derived from the frame synchronizing signals of the system the switch operates to render the appropriate video amplifier 1, 2, 3 conductive to pass the selected source (channel 1, 2, 3) to the transmission channel 4. The video amplifiers 1, 2, 3 may be pentode valves, the electronic switches being effective to supply a positive potential to the suppressor-grids thereof to render them conductive, and to ensure that only the selected source is connected through to the transmission channel at any one time, the electronic switches not appropriate to the selected source (and the corresponding video amplifiers) are rendered inoperative by means of a pulse derived, via a delay line, from the pulses A (Fig. 3, not shown). To prevent spurious signals from appearing in the transmission channel (caused by switching the video amplifiers through a high-impedance circuit, i.e. the suppressor-grid circuit, so that filters cannot be employed without impairing the quality of the video signals), a D.C. clamp-type circuit which is operated during both line and frame blanking intervals is included in the input to the transmission channel (Fig. 3, not shown). Instead of this latter method the arrangement shown in Fig. 6 in which each channel, the last stage of each of which comprises a cathode follower 1<SP>1</SP>, 2<SP>1</SP>, 3<SP>1</SP> having low-resistance cathode loads R4, R5, R6, is connected to the transmission channel 4 through the contacts of relays R61, R62, R63 may be employed. In this arrangement the relays are operated by coils L61, L62, L63 having a resistance approximately equal to that of the cathode load resistances R4, R5, R6 and connected in the anode circuits of electronic switches 61, 62, 63 respectively the normal anode current of each of which is insufficient to operate the relay contacts but, when a switch has been operated (i.e. on the occurrence of the pulse A following the conditioning of one of the switches by operation of the appropriate push-button in the D.C. control unit 21) this current, together with the current due to the control potential from unit 21, is sufficient to " hold " the relay until the removal of the latter potential. Since in this case the switching circuits are of low impedance the introduction of spurious signals is prevented. By employing two independent units of the type described, each feeding into parallel connected gain controllable amplifiers connected to the output transmission channel 4, video mixing (i.e. slow fading of one channel and slow building up of another channel) may be effected. Specification 615,638 is referred to.